Dr. Hanna Joos
Institut f. Atmosphäre und Klima
Phone: +41 44 632 93 65
Senior Scientist in the Atmospheric dynamics group of Prof. Dr. Heini Wernli
My main research interest is on the diabatic modification of the atmospheric flow with a special focus on warm conveyor belts.
Warm conveyor belts (WCBs) are strongly ascending airstreams within extratropical cyclones. They originate in the warm sector of the cyclone and ascend until the upper troposphere. During the ascent, clouds are forming. Thus, WCBs can be seen on satellite pictures as an elongated cloud band ahead of the cold front (see figure 1). During the cloud formation many microphysical processes like condensation, freezing or depositional growth (direct transfer from gas to solid) occur. These processes lead to the release of latent heat which further enhances the WCB ascent.
The latent heat release in the ascending airstream can modify the potential vorticity (PV). In a first order, PV is produced below the maximum heating and destroyed above. Thus, a WCB produces a positive PV anomaly in the mid-troposphere and a negative anomaly in the upper troposphere. These PV anomalies which are produced or enhanced by microphysical processes have the potential to modify the large-, as well as the meso-scale flow and might also be important for the evolution of the cyclone.
In figure 2, trajectories representing a WCB in January 2009 (same WCB as on satellite picture) are shown.
In figure 3, a vertical cross section through the WCB at 38°N is shown. The results are obtained with the NWP-model COSMO. The colors denote the latent heat release that occurs in a WCB due to the different microphysical processes. It can be seen that the main processes contributing to the latent heating are the condensation of cloud water and the depositional growth of snow. Further interesting features are the cooling regions below the cloud. The cooling regions form when the sedimenting hydrometeors, rain or snow, start to sublimate/evaporate below cloud base in sub-saturated air. Thus, the WCB does not only produce areas with strong latent heat release but also areas where a strong cooling occurs.
Another research interest is on cirrus clouds. These are clouds that consist purely of ice crystals and have the potential to strongly modify the Earth radiative budget. Depending on their microphysical properties like the ice water content or the ice crystal number concentration, they can lead to a cooling or warming. The microphysical properties are in turn determined by the dynamical forcing and the thermodynamical environment. The vertical velocity under which the cloud forms, strongly determines the ice crystal number concentration. The relative humidity with respect to ice has also an influence on the ice crystal number concentration but also on the ice water content of the cloud. This complicated interaction of dynamical and thermodynamical processes on different scales is not fully understood. Therefore it is important to investigate the formation mechanisms of cirrus clouds as well as their representation in climate models.
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